A lithographic process is one that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. Stepping and/or scanning movements can be involved, to repeat the pattern at successive target portions across the substrate. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay (the accuracy of alignment between patterns formed in different patterning steps, for example between two layers in a device) and defocus of the lithographic apparatus. Recently, various forms of scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a “spectrum” or “pupil image” from which a property of interest of the target can be determined. Determination of the property of interest may be performed by various techniques: e.g. reconstruction of the target structure by iterative approaches such as rigorous coupled wave analysis or finite element methods; library searches; and principal component analysis.
Examples of scatterometers include angle-resolved scatterometers of the type described in United States patent application publication nos. US 2006-033921 and US 2010-201963. The targets used by such scatterometers are relatively large, e.g. 40 μm by 40 μm, gratings, and the measurement beam generates a spot that is smaller than the grating (i.e., the grating is underfilled). In addition to measurement of feature shapes by reconstruction, diffraction based overlay can be measured using such apparatus, as described in United States patent application publication no. US 2006-066855. Methods and scatterometers are also disclosed in United States patent application publication nos. US 2011-0027704, US 2006-033921 and US 2010-201963. With reduction of the physical dimensions in lithographic processing, there is demand to inspect smaller and smaller features, and also demand to reduce the space occupied by targets dedicated to metrology. The contents of all these applications are incorporated herein by reference.
In order to, e.g., increase the range of scattering angles that can be captured, a solid immersion lens (SIL) or miniature SIL (micro-SIL) can be provided between an objective lens and the target structure. An example of an angularly resolved scatterometer comprising a solid immersion lens (SIL) is disclosed in United States patent application publication no. US 2009-316979. The extreme proximity of the SIL with the target results in a very high effective NA larger than 1, meaning that a greater range of scattering angles can be captured in the pupil image. The application of such a SIL in an inspection apparatus for semiconductor metrology is disclosed in United States patent application publication no. US 2016-061590.
To take advantage of the increasing numerical aperture, the gap between the SIL and the target needs to be set and maintained to an optimal value. For example, the gap may be a few tens of nanometers, for example within the range 10-100 nm to maintain the SIL in the near field of optical interaction with the substrate. Arrangements for controlling the height of the SIL element are described in the United States patent application publication no. US 2016-061590 and in PCT patent application no. PCT/EP2016/058640, filed Apr. 19, 2016. The contents of all the mentioned applications and patent application publications are incorporated herein in their entirety by reference. The use of a SIL can allow formation of a smaller illumination spot, and consequently may also allow the use of smaller targets.